1. Field of the Invention
The present invention generally relates to cardiac pacemakers, and more particularly, pertains to cardiac pacemakers of the type which measure physiologic or metabolic requirements and vary the rate of the pacemaker in accordance therewith and store data related to a variety of physiologic sensor data, sensed and paced events, rates, sensitivities, trends, and the like, for telemetry out on command.
2. Description of the Prior Art
Early cardiac pacemakers provided a fixed-rate stimulation pulse generator that could be reset on demand by sensed atrial and/or ventricular depolarizations. Modern pacemakers include complex stimulation pulse generators, sense amplifiers and leads which can be configured or programmed to operate in single or dual chamber modes of operation, delivering pacing stimuli to the atrium and/or ventricle at fixed rates or rates that vary between an upper rate limit and a lower rate limit.
In recent years, single and dual chamber pacemakers have been developed which measure rate control parameters (RCP's) which are directly or indirectly related to metabolic requirements (e.g., demand for oxygenated blood) and vary the pacing rate in response to such measured RCP's. Such RCP's include, for example, physical activity of the body, right ventricular blood pressure and the change of right ventricular blood pressure over time, venous blood temperature, venous blood oxygen saturation, evoked response, respiration rate, minute ventilation, and various pre- and post-systolic time intervals measured by impedance or pressure sensing within the right ventricle of the heart. Such RCP-measuring, sensor-driven pacemakers have been developed for the purpose of restoring rate response to physiologic stress in patients lacking the ability to increase rate adequately by stress.
In general, a rate responsive pacemaker includes a sensor which produces a sensor output representative of a selected RCP, such sensor output varying between a maximum sensor output level and a minimum sensor output level ("Sensor Output"). The pacemaker provides a pacing rate ("Pacing Rate") which typically varies as a linear or monotonic function ("f") of the sensor output between a selectible lower pacing rate ("Lower Rate") and upper pacing rate ("Upper Rate"). Function f has a selectible slope (i.e., Pacing Rate change/Sensor Output change) adjustable by means of an external programmer in conjunction with the Lower and Upper Rates. Thus, the Pacing Rate typically provided is equal to the pre-selected Lower Rate plus an increment which is a function of the measured Sensor Output, as follows: EQU Pacing Rate=Lower Rate+f (Sensor Output).
A human's heart rate, however, is normally controlled by a complex set of inputs to the autonomic nervous system. Consequently, no single type of sensor has been found to be entirely satisfactory for controlling rate response functions. Some of the shortcomings of single-sensor, rate responsive pacemakers, for example, can include: (1) long-term sensor instability, such as from degradation; (2) long-term changes in correlation between sensor output and its RCP being measured, due to physiologic changes in the patient, such as biologic/sensor interface changes due to tissue changes; (3) changes in sensor sensitivity; and (4) the need for frequent re-programming to accommodate the foregoing problems, as they are encountered.
Various efforts have consequently been made to develop a multiple-sensor pacemaker which is capable of varying its rate as a function of more than one type of measured RCP. Unfortunately, implementation of such multiple sensor-driven rate response concepts has proven to be very difficult and not entirely satisfactory. In addition to those problems listed above as to single-sensor pacemakers, other problems which are typically encountered include: (1) differences between sensors in long-term stability; (2) differences between sensors in immunity to noise; (3) differences in response time to changing metabolic conditions; (4) differences between sensors in correlation between each sensor output and its RCP being measured; and (5) complex set-up procedures, including the need for frequent re-programming.
The enhanced functional capabilities of RCP-measuring, sensor driven process has, together with advances in microprocessor technology, led to the need for and the ability to implement diagnostic data storage of a number of pacing parameters and physiologic event data categories, including the following:
______________________________________ Stored Data for Telemetry Out Prior Art ______________________________________ PVC's (R-sense events) U.S. Pat. No. 4,388,927 PVC couplets U.S. Pat. No. 4,388,927 Pace outputs (events) U.S. Pat. No. 4,388,927 PAC's (P-Sense events) U.S. Pat. No. 4,388,927 P-Sense events during U.S. Pat. No. 4,388,927 P-refractory Loss of Capture (LOC) events U.S. Pat. No. 4,388,927 Noise (R sense) during U.S. Pat. No. 4,388,927 R-refractory Pacemaker reversions for U.S. Pat. No. 4,388,927 V & A sense amp. PAC's U.S. Pat. No. 4,388,927 Trends derived from changes U.S. Pat. No. 4,388,927 from initial test data stored by physician in RAM reflecting above data parameters Bradycardia pauses Nappholz U.S. 4,393,874 Rolling or frozen rate Medtronic .RTM. ELITE .RTM. pacer versus time histograms (trends) P-Sense, R-Sense events Medtronic .RTM. ELITE .RTM. pacer Sensor events Medtronic .RTM. ELITE .RTM. pacer Short, medium and long-term Medtronic .RTM. ELITE .RTM. pacer data periods Rate range histograms in Medtronic .RTM. ELITE .RTM. pacer 8 bins Times of events (PVCs, PACs) U.S. Pat. No. 4,513,743 A-V interval histograms U.S. Pat. No. 4,513,743 Pacing rate or interval U.S. Pat. No. 4,513,743 histograms V-PVC interval histograms U.S. Pat. No. 4,513,743 Peak QRS amplitude histograms U.S. Pat. No. 4,513,743 Pace pulse amplitude, width, U.S. Pat. No. 4,513,743 interval histogram Overflow accumulation of U.S. Pat. No. 4,513,743 histogram bins Alternate accumulation of U.S. Pat. No. 4,513,743 "n" histograms ("previous" and "current") over the same period of time Accumulation of histograms U.S. Pat. No. 4,513,743 for each x hour period of each 24 hour day Sensing threshold U.S. Pat. No. 4,827,934 Percent pacing over "x" time U.S. Pat. No. 4,726,380 period or number of events Average rate over "x" time U.S. Pat. No. 4,726,380 period or number of events Maximum rate over "x" time U.S. Pat. No. 4,726,380 period or number of events Number of tachycardia U.S. Pat. No. 4,726,380 episodes over "x" time period or number of events Maximum tachycardia duration U.S. Pat. No. 4,726,380 over "x" time period or number of events Number of days since implant U.S. Pat. No. 4,726,380 Number of days since last U.S. Pat. No. 4,726,380 reprogramming/data read out Patient medical history and U.S. Pat. No. 4,726,380 medication Programmed parameter values U.S. Pat. No. 4,726,380 Number of high rate episodes Intermedics .RTM. Cosmos .RTM. II sensed Duration of high rate Intermedics .RTM. Cosmos .RTM. II episodes sensed Shortest interval in longest Intermedics .RTM. Cosmos .RTM. II high rate episode Number of PVCs Intermedics .RTM. Cosmos .RTM. II Number of A-sense followed Intermedics .RTM. Cosmos .RTM. II by V-sense Number of A-sense followed Intermedics .RTM. Cosmos .RTM. II by V-pace Number of A-pace followed Intermedics .RTM. Cosmos .RTM. II by V-sense Number of A-pace followed Intermedics .RTM. Cosmos .RTM. II by V-pace Number of times VURL reached Intermedics .RTM. Cosmos .RTM. II Number of times tachy Intermedics .RTM. Cosmos .RTM. II termination initiated Stimulation Thresholds Vitatron Quintech .RTM. TX 915 Short-term rate histogram Siemens Sensolog .RTM. 703 (frozen bins) Sensor Indicated Rate Siemens Sensolog .RTM. 703 Histogram (whether or not pacer is inhibited) every 16 seconds distributed in 4 rate bins, all frozen when one bin filled Pace event counter Siemens Sensolog .RTM. 703 Interference event counter Siemens Sensolog .RTM. 703 Retrograde P-wave termination ELA Chorus .RTM. attempts Fallback starts ELA Chorus .RTM. Number of program changes ELA Chorus .RTM. Other data/events as listed ELA Chorus .RTM. above ECG signal slope/changes as U.S. Pat. No. 4,716,903 time changes ______________________________________
In the context of such RCP-measuring sensor-driven pacemakers, the storage for telemetry out on command of these and a number of other parameters, falling within the categories of current operating and measured parameter values and cumulative logged data is of importance to physicians. For example, it would be desirable to a physician to know just when certain logged data event and histogram bin counters overflow and data collection is halted. In conjunction with sensing natural cardiac depolarization, i.e. P-sense and R-sense events, it is desirable to track the sensing threshold and develop trend data over time. Where the available RCP sensor is capable of detecting capture, it is desirable to record certain operating parameters upon detection of loss-of-capture (LOC).